I use the word functional instead of usable or controllable because that’s kind of under the umbrella of how they could be implemented.
Both Fallout 3 and Fallout 4 had rail systems you could see moving (the presidential metro and Nuka-Express) opposed to Fallout New Vegas (Camp McCarran’s monorail) for example.
I feel like the two major concerns would be how their addition would effect map design and how the engine is going to handle them.
It would be fun to be able to drive around with a sketchy-ass motorcycle. Also bicycles for when you’re a lower level lol. But no having a motorcycle that, unless it’s in perfect working order, that’s a good chance of launching you over the handlebars would be hilarious.